The present invention relates generally to valves, and more particularly to a variable port valve plug that can effectively vary the port size of a valve.
Valves are used in a wide range of process control system applications to control various parameters of a media being processed such as a fluid. A wide variety of valve types are known and can include, for example, dump valves, control valves, throttling valves, and the like. Similarly, process and control systems are utilized for handling a myriad of different media.
A typical valve has a media inlet coupled through a flow control or orifice region to a media outlet. A closure device of some kind is typically provided in the flow control region with a portion that is movable to control flow of a media from the valve inlet to the valve outlet. The movable portion is often a valve plug that can be moved to bear against a corresponding fixed seating surface of the closure device to selectively shut off flow of a media through the valve. During operation, it is typical for a control system to operate the valve so that the valve plug is moved toward and away from the valve seat ring between fixed closed and open positions, respectively, in order to control flow through the orifice region and, thus, through the valve.
One type of valve is a sliding stem valve that has a valve plug on the end of an elongate stem that slides along its longitudinal axis toward and away from the valve seat. A combination of the valve plug, valve seat, and valve stem defines what is known as a trim set for a given valve. A valve with a given flow orifice size and geometry has a maximum flow capacity or flow rate. This flow rate can be related to what is termed xe2x80x9cflow coefficientxe2x80x9d (Cv) used to predict the flow rate through the valve under a standard set of test conditions. The value Cv is the number of U.S. gallons per minute of 60xc2x0 F. water that will flow through the known valve with a pressure drop of 1 pound per square inch (psi) across the valve.
It is known that a given part of a process control system may require a valve having a particular flow rate or flow capacity. It is further known that other portions of the system may require valves with different flow capacities or flow rates. It is also known that a completely different system may require a valve with a still different flow rate or flow capacity. It is also known that the desired flow rate or flow capacity may change for a given process control system when system parameters change. Thus, valve performance characteristics of the particular system may periodically need to be altered. For all of these circumstances, a variety of valve assemblies and/or valve trim sets must be kept on hand.
In order to achieve changeover from one valve flow rate or capacity to another, one must either remove the entire existing valve and replace it with a completely different valve, or rebuild the existing valve to include a different trim set. For a sliding stem type valve, the valve seat typically defines the flow orifice size, and results in selection of a valve plug size that corresponds to that orifice size. The valve plug typically can only move between the valve closed position and a single, fixed valve open position. Each valve thus can only produce one valve flow rate or capacity. As noted above, in order to change from one flow capacity to another, it is necessary to either change the valve trim set or replace the entire valve assembly.
Significant down time, labor expense, and part cost can occur for a process control system as a result of handling multiple valve assemblies and/or trim sets. This is because it is necessary to manufacture, ship, store, handle, and replace the various trim sets or entire valves when a change in valve characteristics is desired. These disadvantages reduce labor, manufacturing, storage, part handling, and facility efficiency and increase labor cost for a given process control system application where any change in valve flow characteristics is desired or necessary.
In addition, if a conventional valve plug and seat set in a valve were to be used to change the flow capacity of the valve, such as by changing the maximum available travel of the valve plug, flow characteristics through the orifice region would be severely and negatively affected. The characteristics of a conventional trim set are not intended for such applications. Also, if a reduced valve flow capacity using a conventional trim set is attempted by stopping and holding the valve plug closer to the seat in a different valve open position, media flowing through the valve would tend to force the plug toward the valve seat because the plug would be positioned too close to the seat. This would cause bi-stable flow and vibration in the valve assembly.